Logically & realistically, wouldn't it likely be some level of mechanical enhancement vs biological doping? No one is testing positive these days (remember back in Armstrong's heyday, so many were getting busted for doping that it wasn't even funny). No one is blowing their ABP. And no doping rings busted like Operation Aderlass was several years ago (and that wasn't even involving any big name cyclists).
And with the ABP, it's so advanced now with the hematological, steroidal & hormonal modules, that at the first sign of any anomalies, there's aggressive target testing like crazy. And the ABP wouldn't allow industrial-size O2-vector doping anyway- the kind of doping that produced those insane performances back in the wild 1990s & early 2000 when riders could dope with impunity. All a rider can really do with the hematological module in play is "microdosing" to stay within their set parameters which doesn't increase blood values that significantly.
And the notion of some undetectable super PED(s) is ridiculous since they freeze samples for up to 8 yrs for retesting. The Russians tried pulling that stunt with some undetectable PEDs at the 2012 London Olympics only to have samples come up positive years later upon retesting. Lol. I can't imagine any riders using some currently undetectable PED(s) knowing their samples with be stored for years to come subject to future retesting.
Perhaps "motordoping" is widespread with some money teams having an advanced R&D program, and the UCI knows about it but doesn't want to open up a can of worms ultimately risking the destuction of the sport?
Just food for thought....